Method for positioning a welding bolt and bolt welding head

ABSTRACT

The invention relates to a method for positioning a welding bolt ( 15 ), held in a bolt welding head ( 3 ), whereby the robot ( 5 ), to which the bolt welding head ( 3 ) is fixed, moves the bolt welding head sideways until a touch sensor ( 21 ) contacts a side wall ( 25 ) of a workpiece. The position of the side wall ( 25 ) is thus detected and the bolt welding head ( 3 ) can be moved through a set amount (Y) into a welding start position, from whence the welding bolt ( 15 ) can be welded to the workpiece ( 9 ) by the displacement in the direction of travel (V). The welding bolt ( 15 ), thus, always has the required separation (Z) from the side wall ( 25 ), independent of tolerances in the workpiece.

This application claims priority to International Patent Application No.WO 01/62426 filed on Aug. 30, 2001 and to German Patent Application No.100 07 837.0 filed on Feb. 21, 2000.

The invention concerns a process for positioning a welding bolt that ismounted on a bolt welding head whereby the bolt welding head is designedfor a programmable multiaxial operating mechanism and whereby thewelding bolt is delivered during welding in a feed direction to aworkpiece onto which the bolt is being welded.

The invention also concerns a bolt welding head, in particular forexecuting movement described by the process, with a welding axle capableof moving in an axial direction, a welding bolt retainer connected tothe welding axle and a contact recognition device that ascertainscontact between the welding bolt and the workpiece during operation ofthe welding axle in a feed direction.

Until the present time, welding bolts have been welded onto theworkpiece as follows:

The multiaxial operating mechanism, usually a programmable industrialrobot, brings the bolt welding head close to the workpiece surface andinto the so-called welding starting position. From this welding startingposition, the bolt welding head moves the welding bolt in the feeddirection until the welding bolt comes into contact with the workpiecesurface. A contact circuit is activated upon contact between the weldingbolt and the workpiece to determine the position of the workpiece in thefeed direction. This position is saved and the welding bolt is raised acertain distance from the workpiece. When the welding bolt is raised,stroke ignition generates an arc and the welding bolt is moved from thebolt welding head, path or speed controlled, for example, into the feedposition and dipped in the melt.

Automobile manufacturing requires the help of robots for welding boltsin flanges, depressions or grooves on the workpiece. The position of thewelding bolt is very important for this process because significantdeviation in the position of the welding bolt can lead to major problemsduring subsequent applications of the welded bolt (e.g. uponinstallation of mountings, clips or metal strips). Because the body of avehicle, for example, on which the welding bolt will be attached hasposition tolerances for transport system's clamping device in theautomobile factory and because the body itself can, in part, havesignificant tolerances, the situation of the nominal position of thewelding bolt moves around in the space causing the welding bolt to bewelded too far from the side walls in flanges, depressions or grooves.

The invention creates a process which positions the welding boltprecisely and creates a bolt welding head that positions the bolt on theworkpiece in a more exact manner.

The invention process encompasses the following steps to achieve thisgoal:

a) the bolt welding head is brought to the workpiece by the operatingmechanism;

b) the operating mechanism moves the bolt welding head diagonal to thefeed direction, to which the welding bolt will be moved during laterwelding, until a contact recognition device designed for the boltwelding head makes contact with a side surface of the workpiece;

c) the operating mechanism moves the bolt welding head diagonal to thefeed position by a specific nominal measurement into a welding startposition; and

d) the welding bolt will be moved in the feed direction until contact ismade with the workpiece surface.

Preferably, an arc between the welding bolt and the workpiece will thenbe generated by stroke ignition when the welding bolt is raised from theworkpiece surface.

During the invention process, not only the axial position of theworkpiece is determined, but also the radial position of the workpieceby detecting a side surface of the workpiece and determining itsposition. This makes it possible to achieve the exact nominal distance(distance of the side surface from the welded bolt) and to position thewelding bolt precisely in flanges, depressions or grooves in theworkpiece.

Further development of the invention includes moving the operatingmechanism of the bolt welding head in step b) in the first directionsideways toward a side surface of the workpiece, whereby a contactrecognition device on the bolt welding head signals that contact hasbeen made with the workpiece and whereby the first indicated positioncontact with the workpiece is recorded and saved. The bolt welding headis then moved in a second direction set opposite to the first directionuntil it reaches a second position where the contact recognition devicesno longer signals that contact is being made with the workpiece. Thesecond position is recorded and saved. A reference value from the firstand second position is then determined from which the bolt welding headis moved sideways by the pre-determined nominal measurement into thewelding starting position.

The reference value is preferably the average between the first andsecond position. The reference value determination is based on the boltwelding head approaching the side surface at a specific speed and aspecific temporal hesitation occurring until the bolt welding headremains stationary in the first position so as to run over the sidesurface, so to say, and the first position does not yield the exact sideposition of the side surface. Also, when moving in the second direction,a temporal hesitation occurs until the contact recognition devices nolonger signals contact and/or the bolt welding head remains stationaryin the second position. The reference value from the first and secondposition is then the fictitious actual position of the side surface. Thereference values can depend on the speeds that are used to move the boltwelding head in the first and second direction.

In addition, step d) includes the bolt welding head moving the weldingbolt, whereby it is also possible for the multiaxial operating mechanismto bring the bolt welding head relatively close to the workpiece surfaceand the bolt welding head itself then moves the welding axle in the feeddirection to achieve contact with the workpiece.

The preferred execution form for the invention process is to use acontact recognition device that determines contact with the workpiece instep d) upon movement in the feed direction to indicate contact with theworkpiece in step b) (movement diagonal to the feed position). Thismeans that no significant supplemental effort needs to be made toachieve side position recognition.

The bolt welding head using in the invention is characterized by havinga contact recognition device that determines the presence of a sidesurface on the workpiece upon moving the bolt welding head diagonal tothe feed direction. Diagonal to the feed position is preferably at aright angle to the feed direction.

The contact recognition device used to detect the side surface uses aprobe to touch the side surface in the preferred execution form.

This probe should be situated elastically with or without a spring onthe bolt welding head so that it is not distorted plastically when itcomes into contact with the side surface of the workpiece.

According to the preferred execution form, the probe comprises a casesurrounding the welding bolt. This has the advantage of enabling theprobe to move in any direction and to contact the side surface of theworkpiece in front of the welding bolt. Preferably the exterior surfaceof the case is concentric to the welding axle so that there isconstantly an equal distance between the exterior case surface, whichproduces contact with the side surface, and the axis of the welding boltindependent of movement direction when determining the position of theworkpiece.

In addition, the contact recognition device for determining contactbetween the welding bolt and the workpiece is also the contactrecognition device that has the probe so that only one contactrecognition device is needed, which serves to recognize position in thefeed direction and in the sideways direction.

It is advantageous to fasten the probe on the welding axle and provideit with electrical current. To prevent it from being welded with thewelding bolt, it will not be in direct contact with the bolt, wherebyboth parts used in position recognition are linked with each otherelectronically.

Other characteristics and advantages of the invention are contained inthe following description and from the following illustrations, whichwill be referenced. The illustrations show:

FIG. 1 a bolt welding head as described in the invention and theworkpiece on which a welding bolt is to be welded;

FIG. 2 a cutaway view along the line II—II in FIG. 1:

FIG. 3 the bolt welding head during position recognition in a differentworkpiece environment; and

FIG. 4 an enlarged view of the front end of the welding bolt with acutaway case.

FIG. 2 displays the front part of a bolt welding head that is fastenedto a programmable multiaxial operating mechanism 5 in the form of anindustrial robot. Using the bolt welding head, welding bolts 15 are tobe welded in a depression 7 on a workpiece 9 in the form of a vehiclemold. The bolt welding head has linear drive that is not shown ingreater detail, which can move a welding axle 11 with a welding boltretainer 13 fastened Onto it in an axial direction, i.e., feed directionV and the opposing direction R.

FIG. 1 displays a welding bolt 15 contained in the welding bolt retainer13 that is also displayed having been fastened in a welded position onthe workpiece. The welding bolt retainer 13 is surrounded by a probe inthe shape of a rounded cylindrical case 17 slit several times along itslength whose exterior side wall 21 runs concentric to axis A andconcentric to the welding axle (compare FIG. 2). The case 17 is onepiece fastened with a cover nut 51 that serves to lock the welding boltretainer 13 to the welding axle 11 (FIG. 4).

The welding bolt retainer 13 and the welding bolt 15 are in the feeddirection V opposite the case 17.

The welding bolt retainer 13 and the case 17 are connected electricallywith a common contact recognition device 23 so that both are exposed tothe same voltage.

The contact recognition device 23 that can be integrated with a contactcircuit or a steering unit of the bolt welding head, for example, servesto determine the axial and radial position of the workpiece surface.

The case 17 can be closed in the perimeter direction. There can also bea number of finger-like extensions or a peg-shaped probe that isconnected to the contact recognition device 23.

The bolt welding head displayed is adapted to weld the welding bolt 15at a specific distance Z away from the side wall 25 of the workpiece 9.The distance Z is the so-called nominal distance. This nominal distanceis supposed to be maintained by placing the welding bolt 15 in thedepression 7 in a precise manner.

In order to weld the welding bolt 15 in its exact position, thefollowing process is used:

a) The bolt welding head 3 is moved into position by the industrialrobot 5 close to the workpiece, i.e., in a position in a sidewaysdirection that corresponds approximately to the position of the weldedbolt, which was programmed without occurring tolerances in the space asa reference value.

b) The robot 5 moves the bolt welding head 3 at a right angle to thefeed direction V, in direction Bi to the case 17 that is part of thecontact recognition device whose exterior side surface contacts the sidewall 25. The contact recognition device 23, which can also act as a typeof switch, sends a corresponding signal to the steering unit that is notshown, which immediately stops the industrial robot 5. The firstposition in which the bolt welding head is located is recorded andsaved. Because the distance of axis A on the welding bolt 15 is knownfrom the exterior cover surface 21 of the case 17 (Measurement X), theside position of the side surface of the side wall of axis A can bedetermined.

c) The robot 5 now moves the bolt welding head 3 in a second directionB2 that is set opposite to direction B1 in which the bolt welding head 3was moved to the side wall 25. The bolt welding head 3 is moved indirection B2 by the nominal measurement Y (nominal distanceZ-Measurement X) so that axle A is located at nominal distance X fromthe side wall 25.

d) Then the industrial robot 5 moves the welding bolt 15 in direction Vclose to the workpiece. The linear motor moves the welding axle 11 andthereby the welding bolt 15 in direction V until the welding until thewelding bolt 15 comes into contact with the surface of the workpiece 9.The contact recognition device 23 reacts, the position of the weldingaxle 11 is determined using a path measurement system in the boltwelding head 3.

e) With the path controlled, welding axle 11 is raised in direction R bya predetermined distance from the workpiece surface.

f) Raising causes stroke ignition to generate an arc between the weldingbolt 15 and workpiece 7.

g) Welding axle 11 is moved in a steered away position in direction Vand the welding bolt 15 is dipped in the welding material that forms.

Welding bolt 15 is now fixed in its precise position on the workpiece.

The case 17 is connected through the slit electrically with the boltretainer 13 and sprung radially. Touching of the workpiece is detectedusing a contact message (electrical current between the case 17 or thewelding bolt 15 and the workpiece).

FIG. 3 displays another start situation for the bolt welding head wherethe welding bolt 15 must be attached in a horizontal direction to a wallthat is not positioned in a flange or a depression. Here the boltwelding head is moved diagonal to the feed direction V far enough so thecovering of the side wall 25 is contacted by the case 17. Then the boltwelding head will be moved back by the industrial robot in direction Rso that the welding bolt 15 does not hit the side wall 25 duringmovement into the welding start position in direction B1. Then the boltwelding head 3 will move in direction B1 by a nominal measurement Y(Y=X+Should-distance Z) into the welding start position. From there,either the robot can move the bolt welding head 3 closer to the surfaceof the workpiece or the bolt welding head moves the welding axel 11 indirection V until the welding bolt 15 comes into contact with theworkpiece 9. The following steps are the steps for welding the boltalready described in connection with FIG. 1:

FIG. 3 displays another start situation for the bolt welding head wherethe welding bolt 15 must be attached in a horizontal direction to a wallthat is not positioned in a flange or a depression. Here the boltwelding head is moved diagonal to the feed direction V far enough so thecovering of the side wall 25 is contacted by the case 17. Then the boltwelding head will be moved back by the industrial robot in direction Rso that the welding bolt 15 does not hit the side wall 25 duringmovement into the welding start position in direction B1. Then the boltwelding head 3 will move in direction B1 by a nominal measurement Y(Y=X+distance Z) into the welding start position. From there, either therobot can move the bolt welding head 3 closer to the surface of theworkpiece or the bolt welding head moves the welding axle 11 indirection V until the welding bolt 15 comes into contact with theworkpiece 9. The following steps are the steps for welding the boltalready described in connection with FIG. 1:

When the bolt welding head 3 is moved sideways and the case 17 contactsthe side surface of the workpiece, there will be certain hesitationuntil the robot 5 comes to a stop. The case 17 is moved sidewaysslightly in this first position so that the Measurement X does notcorrespond exactly to the distance of the axis from the side surface,rather it is greater than the actual distance. To correct this slighterror, the bolt welding head will be moved in a second direction B2 setopposite to direction B1 in accordance with a modified movement untilthe contact recognition device 23 no longer signals contact with theworkpiece. The robot 5 will immediately stop and will record and savethe second position. The reference value will be determined from thefirst and second position, preferably the average between bothpositions. Based on this middle value, the bolt welding head 3 will bemoved by the nominal measurement Y into the welding start position fromwhich the welding bolt 15 will be moved only in an axial direction untilit is welded.

When the side surfaces to be detected do not stand at a right angle tothe movement direction B1 and the position of the workpiece can sway inthe process, this could lead to inaccuracies when determining theposition of the side surfaces. For this reason, the welding head my beset first in the feed direction V until the bolt touches the workpieceand the position of the workpiece in feed direction V is determinedfirst. Then the bolt will be raised by a defined amount in direction Rand finally, as explained, will move sideways to determine the positionof the side surfaces.

1. A method of locating and welding a weld stud on a target surface of aworkpiece having an adjacent side surface, wherein the weld stud ismounted in a stud welding head having a position recognition devicemounted on a programmable multiaxial operating mechanism, said methodcomprising the following steps: moving the stud welding head axially ina feed direction, bringing the stud welding head to the workpiece;actuating the programmable multiaxial operating mechanism to move thestud welding head laterally in a first direction perpendicular to thefeed direction to contact the side surface of the workpiece; actuatingthe programmable multiaxial operating mechanism to move the stud weldinghead laterally in a second direction generally opposite to the firstdirection a predetermined distance to a predetermined start weldposition; and moving the stud welding head axially in the feed directionto bring the weld stud into contact with the target surface of theworkpiece.
 2. The method as defined in claim 1, wherein the positionrecognition device signals contact between the stud welding head and theside surface of the workpiece as a first position of the stud weldinghead, wherein the first position is recorded and saved by a control,moving the stud welding head in the second direction opposite the firstdirection to a second position, where the position recognition deviceindicates that the weld stud head is no longer in contact with the sidesurface of the workpiece and the second position is recorded and saved,then continuing to move the stud welding head in the second direction tothe predetermined welding start position based upon the first and secondrecorded positions of the stud welding head.
 3. The method as defined inclaim 2, wherein said method includes determining said predeterminedwelding start position by calculating the average between the first andsecond positions of the stud welding head.
 4. The method as defined inclaim 1, wherein said method includes using said contact recognitiondevice to indicate contact between said weld stud and said targetsurface of said workpiece, then actuating said programmable multiaxialoperating mechanism to withdraw said weld stud from said target surfacea predetermined distance and initiating an arc between said weld studand said target surface.
 5. A stud welding apparatus for welding a studon a target surface of a workpiece having an adjacent side surface, saidstud welding apparatus comprising: a programmable multiaxial operatingmechanism; a stud welding head mounted on said programmable multiaxialoperating mechanism including a weld stud retainer adapted to receive aweld stud, an annular case surrounding said weld stud retainer in spacedrelation and a contact recognition device connected to a control of saidprogrammable multiaxial operating mechanism; and wherein saidprogrammable multiaxial operating mechanism moves said stud welding headlaterally in a first direction perpendicular to an axis of said studwelding head to contact said annular case with said side surface of saidworkpiece, generating a signal from said contact recognition device tosaid control, and then moving said stud welding head a predetermineddistance in a second direction opposite to said first direction to apredetermined welding start position.
 6. The stud welding apparatus asdefined in claim 5, wherein said contact recognition device is connectedto said annular case, such that said contact signaling device generatesa signal upon contact of said annular case with said side surface ofsaid workpiece.
 7. The stud welding apparatus as defined in claim 6,wherein said annular case includes a plurality of radial slots, suchthat said annular case is resiliently biased inwardly upon contact withsaid side surface of said workpiece.